Method Of Post-Mold Crosslinking Thermoplastic Polyurethane Golf Ball Cover Compositions

ABSTRACT

A golf ball includes a cover material containing a thermoplastic polyurethane composition which may be crosslinked post-mold by applying a radiation source such as ultraviolet light. The thermoplastic polyurethane composition may be crosslinked under controlled conditions selected to yield golf balls exhibiting desired physical properties and performance characteristics, such as hardness, spin, feel, and distance. In some examples, the golf ball contains one or more additional components, such as a UV curable topcoat, that may be simultaneously treated by the radiation source, thus improving processing efficiency.

BACKGROUND

Golf balls generally have either a one-piece construction or severallayers including an outer cover surrounding a core. A wound ballconfiguration typically has a vulcanized rubber thread wound undertension around a solid or semi-solid core, which is then enclosed in asingle or multi-layer covering of tough, protective material. Anothertype of ball, a one-piece ball, typically formed from a solid mass ofmoldable resilient material which has been cured to develop thenecessary degree of hardness. One-piece molded balls generally do nothave an enclosing cover. Multi-piece (two or more pieces) non-woundballs generally have a solid or liquid core of one or more layers, and acover having one or more layers formed over the core.

Many multi-piece golf balls have a cover containing an ionomer resin toimpart toughness and cut resistance. Examples of such ionomers includeSurlyn®, available from E.I. DuPont de Nemours and Company, and Iotek®,available from Exxon-Mobil.

Polyurethanes also have been used in cover materials of multi-piece golfballs.

Polyurethanes may be formed by mixing two primary ingredients duringprocessing, most commonly a polyisocyanate, e.g., diphenylmethanediisocyanate monomer, toluene diisocyanate, or their derivatives, and apolyol, e.g., a polyester- or polyether polyol. An isocyanate that isreacted with a polyamine forms a polyurea. The term “polyurethane” isoften used to describe polyurethane/polyurea systems. Polyurethanes maybe thermoset, e.g., having a crosslinked molecular structure, orthermoplastic, e.g., having a linear molecular structure. A polyurethanebecomes irreversibly “set” when a polyurethane prepolymer is crosslinkedwith a polyfunctional curing agent, such as a polyamine or polyol. Theprepolymer typically is made from polyether or polyester. Crosslinkingoccurs between the isocyanate groups and the hydroxyl end-groups of thepolyol. The physical properties of thermoset polyurethanes may beadjusted by the degree of crosslinking. For example, tightly crosslinkedpolyurethanes are fairly rigid and strong, whereas a lower level ofcrosslinking results in materials that are flexible and resilient.Depending upon the processing method, reaction rates may be quite fast,e.g., as in the case for some reaction injection molding (RIM) systems,or in other cases may be several hours or longer, e.g., as in severalcoating systems.

SUMMARY

The following presents a general summary of aspects of the invention inorder to provide a basic understanding of the invention and variousfeatures of it. This summary is not intended to limit the scope of theinvention in any way, but it simply provides a general overview andcontext for the more detailed description that follows.

Aspects of this invention are directed to methods of preparingmulti-piece golf balls. In one example, a multi-piece golf ball isprepared by providing a core layer and a cover layer. The cover layerincludes a crosslinkable thermoplastic polyurethane. The cover layer andcore layer are molded into a golf ball preform. Any suitable moldingtechnique may be used such as injection molding, compression molding,retractable pin injection molding, vacuum forming, reaction injectionmolding, liquid injection molding, flow coating, and the like. The golfball preform is removed from the mold. The golf ball preform is thenirradiated under conditions sufficient to crosslink the crosslinkablethermoplastic polyurethane. In some embodiments, one or moreintermediate layers are present between the core layer and the coverlayer, and/or the core may have a multi-layer construction and/or thecore may have a multi-layer construction.

In another aspect, a multi-piece golf ball is prepared by molding acover layer and a core layer to form a golf ball preform in a mold. Thecover layer includes a crosslinkable thermoplastic polyurethane. Thegolf ball preform is removed from the mold and stored for at least oneday. Demand for particular golf ball properties and/or performancecharacteristics are then determined The golf ball preform is thenirradiated to crosslink the crosslinkable thermoplastic polyurethane toachieve the desired golf ball properties and/or performancecharacteristics.

In yet another aspect, a multi-piece golf ball is prepared by providinga core layer and a cover layer. The cover layer includes a crosslinkablethermoplastic polyurethane. The cover layer and core layer are moldedinto a golf ball preform, and removed from the mold. An ultravioletcurable topcoat composition is applied to the exterior surface of thegolf ball preform. The golf ball preform is then irradiated withultraviolet radiation to simultaneously crosslink the crosslinkablethermoplastic polyurethane and cure the UV curable topcoat composition.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and certainadvantages thereof may be acquired by referring to the followingdetailed description in consideration with the accompanying drawings, inwhich:

FIGS. 1 and 1A schematically illustrate a cross-sectional view of amulti-piece golf ball.

FIG. 2 is a process flow diagram illustrating one example of post-moldUV irradiation of thermoplastic polyurethane golf ball covercompositions.

FIG. 3 is a process flow diagram illustrating an example ofpost-distribution UV irradiation of thermoplastic polyurethane golf ballcover compositions.

FIG. 4 is a process flow diagram illustrating an example of simultaneouspost-mold UV irradiation of thermoplastic polyurethane in a covercomposition and a UV curable topcoat.

DETAILED DESCRIPTION

In the following description of various example structures, reference ismade to the accompanying drawings, which form a part hereof, and inwhich are shown by way of illustration various example golf ballstructures. Additionally, it is to be understood that other specificarrangements of parts and structures may be utilized and structural andfunctional modifications may be made without departing from the scope ofthe present invention. Also, while terms such as “top,” “bottom,”“front,” “back,” “rear,” “side,” “underside,” “overhead,” and the likemay be used in this specification to describe various example featuresand elements of the invention, these terms are used herein as a matterof convenience, e.g., based on the example orientations shown in thefigures and/or the orientations in typical use. Nothing in thisspecification should be construed as requiring a specific threedimensional or spatial orientation of structures.

A. General Description of Golf Balls and Manufacturing Systems andMethods

Golf balls may be of varied constructions, e.g., one-piece balls,two-piece balls, three-piece balls (including wound balls), four-pieceballs, etc. The difference in play characteristics resulting from thesedifferent types of constructions can be quite significant. Generally,golf balls may be classified as solid or wound balls. Solid balls thathave a two-piece construction, typically a cross-linked rubber core,e.g., polybutadiene rubber cross-linked with zinc diacrylate and/orsimilar cross-linking agents, encased by a blended cover, e.g., ionomerresins, are popular with many average recreational golfers. Thecombination of the core and cover materials provide a relatively “hard”ball that is virtually indestructible by golfers and one that imparts ahigh initial velocity to the ball, resulting in improved distance.Because the materials of which the ball is formed are very rigid,two-piece balls tend to have a hard “feel” when struck with a club.Likewise, due to their hardness, these balls have a relatively low spinrate off the driver, which also helps provide greater distance.

Wound balls are generally constructed from a liquid or solid centersurrounded by tensioned elastomeric material and covered with a durablecover material, e.g., ionomer resin, or a softer cover material, e.g.,balata or polyurethane. Wound balls are generally thought of asperformance golf balls and have good resiliency, desirable spincharacteristics, and feel when struck by a golf club. However, woundballs are generally difficult to manufacture as compared to solid golfballs.

More recently, three- and four-piece balls have gained popularity, bothas balls for average recreational golfers as well as performance ballsfor professional and other elite level players.

A variety of golf balls have been designed to provide particular playingcharacteristics. These characteristics generally include the initialvelocity and spin of the golf ball, which can be optimized for varioustypes of players. For instance, certain players prefer a ball that has ahigh spin rate in order to control and stop the golf ball around thegreens. Other players prefer a ball that has a low spin rate and highresiliency to maximize distance. Generally, a golf ball having a hardcore and a soft cover will have a high spin rate. Conversely, a golfball having a hard cover and a soft core will have a low spin rate. Golfballs having a hard core and a hard cover generally have very highresiliency for distance, but are hard feeling and difficult to controlaround the greens.

The carry distance of some conventional two-piece balls has beenimproved by altering the typical single layer core and single coverlayer construction to provide a multi-layer ball, e.g., a dual coverlayer, a dual core layer, and/or a ball having an intermediate layerdisposed between the cover and the core (also called a “mantle” layer).Three- and four-piece balls (and even five-piece balls) are now commonlyfound and commercially available. Aspects of this invention may beapplied to all types of constructions, including the various wound,solid, and/or multi-layer ball constructions with any number of layersdescribed above.

FIGS. 1 and 1A show an example of a golf ball 10, which has a core 12,an intermediate layer 14, a cover 16 having a plurality of dimples 18,and a topcoat 20 applied over the exterior surface of the golf ball 10.The ball 10 also may have any other construction, including the variousexample constructions described herein. The thickness of the topcoat 20typically is significantly less than that of the cover 16 or theintermediate layer 14, and by way of example may range from about 5 toabout 25 μm. The topcoat 20 should have a minimal effect on the depthand volume of the dimples 18.

The cover 16 of the golf ball 10 may be made of any number of materialssuch as, but not limited to, ionomeric, thermoplastic, elastomeric,urethane, balata (natural or synthetic), polybutadiene, or combinationsthereof As described below, at least one layer of the cover 16 containsa crosslinkable thermoplastic polyurethane. An optional primer orbasecoat may be applied to the exterior surface of the cover 16 of thegolf ball 10 prior to application of the coating layer 20.

The Center

A golf ball may be formed, for example, with a center having a lowcompression, but still exhibit a finished ball COR and initial velocityapproaching that of conventional two-piece distance balls. The centermay have, for example, a compression of about 60 or less. The finishedballs made with such centers have a COR, measured at an inbound speed of125 ft./s., of about 0.795 to about 0.815. “COR” refers to Coefficientof Restitution, which is obtained by dividing a ball's rebound velocityby its initial (i.e., incoming) velocity. This test is performed byfiring the samples out of an air cannon at a vertical steel plate over arange of test velocities (e.g., from 75 to 150 ft/s). A golf ball havinga high COR dissipates a smaller fraction of its total energy whencolliding with the plate and rebounding therefrom than does a ball witha lower COR.

The terms “points” and “compression points” refer to the compressionscale or the compression scale based on the ATTI Engineering CompressionTester. This scale, which is well known to persons skilled in the art,is used in determining the relative compression of a center or ball.

The center may have, for example, a Shore D hardness of about 65 toabout 80. The center may have, for example, a diameter of about 1.25inches to about 1.5 inches. The base composition for forming the centermay include, for example, polybutadiene and about 20 to 50 parts of ametal salt diacrylate, dimethacrylate, or monomethacrylate. If desired,the polybutadiene can also be mixed with other elastomers known in theart, such as natural rubber, styrene butadiene, and/or isoprene, inorder to further modify the properties of the center. When a mixture ofelastomers is used, the amounts of other constituents in the centercomposition are usually based on 100 parts by weight of the totalelastomer mixture.

Metal salt diacrylates, dimethacrylates, and monomethacrylates includewithout limitation those wherein the metal is magnesium, calcium, zinc,aluminum, sodium, lithium or nickel. Zinc diacrylate, for example,provides golf balls with a high initial velocity in the United StatesGolf Association (“USGA”) test.

Free radical initiators often are used to promote cross-linking of themetal salt diacrylate, dimethacrylate, or monomethacrylate and thepolybutadiene. Suitable free radical initiators include, but are notlimited to peroxide compounds, such as dicumyl peroxide;1,1-di(t-butylperoxy) 3,3,5-trimethyl cyclohexane;bis(t-butylperoxy)diisopropylbenzene; 2,5-dimethyl-2,5di(t-butylperoxy)hexane; or di-t-butyl peroxide; and mixtures thereofThe initiator(s) at 100 percent activity may be added in an amountranging from about 0.05 to about 2.5 pph based upon 100 parts ofbutadiene rubber, or butadiene rubber mixed with one or more otherelastomers. Often the amount of initiator added ranges from about 0.15to about 2 pph, and more often from about 0.25 to about 1.5 pph. Thegolf ball centers may incorporate 5 to 50 pph of zinc oxide (ZnO) in azinc diacrylate-peroxide cure system that cross-links polybutadieneduring the core molding process.

The center compositions may also include fillers, added to theelastomeric composition to adjust the density and/or specific gravity ofthe center. Non-limiting examples of fillers include zinc oxide, bariumsulfate, and regrind, e.g., recycled core molding matrix ground to about30 mesh particle size. The amount and type of filler utilized isgoverned by the amount and weight of other ingredients in thecomposition, bearing in mind a maximum golf ball weight of 1.620 oz hasbeen established by the USGA. Fillers usually range in specific gravityfrom about 2.0 to about 5.6. The amount of filler in the center may belower such that the specific gravity of the center is decreased.

The specific gravity of the center may range, for example, from about0.9 to about 1.3, depending upon such factors as the size of the center,cover, intermediate layer and finished ball, as well as the specificgravity of the cover and intermediate layer.

Other components such as accelerators, e.g., tetra methylthiuram,processing aids, processing oils, plasticizers, dyes and pigments,antioxidants, as well as other additives well known to the skilledartisan may also be used in amounts sufficient to achieve the purposefor which they are typically used.

Intermediate Layer(s)

The golf ball also may have one or more intermediate layers formed, forexample, from dynamically vulcanized thermoplastic elastomers,functionalized styrene-butadiene elastomers, thermoplastic rubbers,thermoset elastomers, thermoplastic urethanes, TPEs, metallocenepolymers, thermoset urethanes, ionomer resins, or blends thereof Forexample, an intermediate layer may include a thermoplastic or thermosetpolyurethane. Non-limiting examples of commercially availabledynamically vulcanized thermoplastic elastomers include SANTOPRENE®,SARLINK®, VYRAM®, DYTRON®, and VISTAFLEX®. SANTOPRENE® is a dynamicallyvulcanized PP/EPDM. Examples of functionalized styrene-butadieneelastomers, i.e., styrene-butadiene elastomers with functional groupssuch as maleic anhydride or sulfonic acid, include KRATON FG-1901x andFG-1921x, which are available from the Shell Corporation of Houston,Tex.

Non-limiting examples of suitable thermoplastic polyurethanes includeESTANE® 58133, ESTANE® 58134 and ESTANE® 58144, which are commerciallyavailable from the Lubrizol Company of Cleveland, Ohio.

Examples of metallocene polymers, i.e., polymers formed with ametallocene catalyst, include those commercially available from SentinelProducts of Hyannis, Mass. Suitable thermoplastic polyesters includepolybutylene terephthalate. Thermoplastic ionomer resins may be obtainedby providing a cross metallic bond to polymers of monoolefin with atleast one member selected from the group consisting of unsaturated mono-or di-carboxylic acids having 3 to 12 carbon atoms and esters thereof(the polymer contains 1 to 50 percent by weight of the unsaturated mono-or di-carboxylic acid and/or ester thereof). More particularly, lowmodulus ionomers such as acid-containing ethylene copolymer ionomers,include E/X/Y copolymers where E is ethylene, X is a softening comonomersuch as acrylate or methacrylate. Non-limiting examples of ionomerresins include SURLYN® and LOTEK®, which are commercially available fromDuPont and Exxon-Mobil, respectively.

Alternatively, the intermediate layer may be a blend of a first and asecond component wherein the first component is a dynamically vulcanizedthermoplastic elastomer, a functionalized styrene-butadiene elastomer, athermoplastic or thermoset polyurethane or a metallocene polymer and thesecond component is a material such as a thermoplastic or thermosetpolyurethane, a thermoplastic polyetherester or polyetheramide, athermoplastic ionomer resin, a thermoplastic polyester, anotherdynamically vulcanized elastomer, another a functionalizedstyrene-butadiene elastomer, another a metallocene polymer or blendsthereof. At least one of the first and second components may include athermoplastic or thermoset polyurethane.

An intermediate layer also may be formed from a blend containing anethylene methacrylic/acrylic acid copolymer. Non-limiting examples ofacid-containing ethylene copolymers include ethylene/acrylic acid;ethylene/methacrylic acid; ethylene/acrylic acid/n- or isobutylacrylate; ethylene/methacrylic acid/n- or iso-butyl acrylate;ethylene/acrylic acid/methyl acrylate; ethylene/methacrylic acid/methylacrylate; ethylene/acrylic acid/iso-bornyl acrylate or methacrylate andethylene/methacrylic acid/isobornyl acrylate or methacrylate. Examplesof commercially available ethylene methacrylic/acrylic acid copolymersinclude NUCREL® polymers, available from DuPont.

Alternatively, an intermediate layer may be formed from a blend whichincludes an ethylene methacrylic/acrylic acid copolymer and a secondcomponent which includes a thermoplastic material. Suitablethermoplastic materials for use in the intermediate blend include, butare not limited to, polyesterester block copolymers, polyetheresterblock copolymers, polyetheramide block copolymers, ionomer resins,dynamically vulcanized thermoplastic elastomers, styrene-butadieneelastomers with functional groups such as maleic anhydride or sulfonicacid attached, thermoplastic polyurethanes, thermoplastic polyesters,metallocene polymers, and/or blends thereof

The intermediate layer often has a specific gravity of about 0.8 ormore. In some examples the intermediate layer has a specific gravitygreater than 1.0, e.g., ranging from about 1.2 to about 1.3. Specificgravity of the intermediate layer may be adjusted, for example, byadding a filler such as barium sulfate, zinc oxide, titanium dioxide andcombinations thereof.

The intermediate layer blend may have a flexural modulus of less thanabout 10,000 psi, often from about 5,000 to about 8,000 psi. Theintermediate layers often have a Shore D hardness of about 35 to 50. Theintermediate layer and core construction together may have a compressionof less than about 65, often from about 50 to about 65. Usually, theintermediate layer has a thickness from about 0.020 inches to about0.125 inches.

The golf balls may include a single intermediate layer or a plurality ofintermediate layers. In the case where a ball includes a plurality ofintermediate layers, a first intermediate layer may include, forexample, a thermoplastic material having a hardness greater than that ofthe core. A second intermediate layer may be disposed around the firstintermediate layer and may have a greater hardness than that of thefirst intermediate layer. The second intermediate layer may be formedof, but not limited to, materials such as polyether or polyesterthermoplastic urethanes, thermoset urethanes, and ionomers such asacid-containing ethylene copolymer ionomers.

In addition, a third intermediate layer may be disposed in between thefirst and second intermediate layers. The third intermediate layer maybe formed of the variety of materials as discussed above. For example,the third intermediate layer may have a hardness greater than that ofthe first intermediate layer. There may also be additional intermediatelayers.

The Cover Layer

A golf ball also typically has a cover layer that includes one or morelayers of a thermoplastic or thermosetting material. A variety ofmaterials may be used such as ionomer resins, polyurethanes, balata andblends thereof. As described herein, at least one of the layers in thecover layer includes a crosslinkable thermoplastic polyurethane (TPU).The crosslinkable thermoplastic polyurethane is initially athermoplastic, and while in this state may be melted and solidifiedrepeatedly. Crosslinking generally increases hardness and, as describedmore fully below, may be controlled to impart desired properties to thecover and thus performance characteristics to the golf ball.

Polyurethanes typically are formed by reacting a polyol with apolyisocyanate. In some cases, the polyisocyanate is in the form of apolyurethane prepolymer formed by reaction between a polyether orpolyester and a polyisocyanate. Two types of polyisocyanates arepredominantly used to make polyurethanes, diphenylmethane diisocyanatemonomer (MDI) and its derivatives, and toluene diisocyanate (TDI) andits derivatives.

MDI is the most widely used polyisocyanate. Both rigid and flexiblefoams, reaction injection moldings, elastomers, coatings, and castingcompounds are made from MDI. There are three basic grades of MDI,polymeric MDI, pure MDI, and pure MDI derivatives. Pure MDI, which isproduced from polymeric MDI, is a low-melting-temperature (about 100°F.) solid. Its primary use is in thermoplastic and cast elastomers. Italso is used as an additive for synthetic fibers to achieve high fibertenacity and elongation.

Pure MDI derivatives may be tailored to provide specific processing andreaction characteristics. A major use for these solvent-free liquids isin reaction injection molding (RIM), but they also find application inintegral skin moldings, semi-flexible moldings, and cast elastomers.Toluene diisocyanate, TDI, is used primarily to make flexible foam, andalso is used in elastomers, sealants, and coatings. TDI's generally arewater-white liquids which have much higher isocyanate (—NCO) contentthan MDI and lower molecular weights. MDI and TDI also may be blended,particularly for producing flexible molded foams. The free-flowing,brown liquid blends usually have nearly as high isocyanate contents asdoes TDI.

There are two main types of polyols used in polyurethanes systems,polyesters and polyethers. Polyols are usually identified by theirfunctionality. Functionality pertains to the number of reactive sites,which controls crosslinking. The more crosslinked (higherfunctionality), the more rigid the polyurethane. Functionality iscontrolled by the initiator used to manufacture the polyol. Glycerine,for example, is commonly used to initiate triol (3 functional) polyols.An oxide such as propylene oxide, ethylene oxide, or a combination, isoften added to the initiator to extend the molecular chain and tailorfinal processing and performance characteristics of the polyol. Triolsoften are used to produce flexible foams, while diols commonly are usedfor elastomers, coatings, and sealants. Tetrols typically are used forrigid foams.

Polyether-based polyols have greater resistance to hydrolysis. Polyetherpolyols may be modified, for example, by in situ polymerization ofacrylonitrile/styrene monomers. The resulting graft polyols generallyproduce flexible foams with improved load-bearing properties as well asgreater tensile and tear strengths. Depending on the backbone on whichthese vinyl monomers are grafted, a wide range of performancecharacteristics may be developed.

Polyester polyols generally yield polyurethanes with greater strengthproperties, wear resistance, and thermal stability than polyetherpolyurethanes, and they can absorb more energy. Polyester polyols aretypically classed by molecular weight. Low molecular weight polyols(e.g., less than 1500) are used in coatings, casting compounds, andrigid foams. Medium molecular weight polyols (e.g., 1550 to 2500) areoften used in elastomers. High molecular weight polyols (e.g., greaterthan 2500) are typically used in flexible foams.

Although conventional TPUs do not readily crosslink, TPUs may be madecrosslinkable by adjusting the chemistry and/or with the addition ofco-agents. See, e.g., Limerkens et al. U.S. 2009/0197000 A1, thedisclosure of which is hereby incorporated by reference in its entirety.Non-limiting examples of commercially available crosslinkable TPUsinclude Elastollan™ 1100, which are polyether-based thermoplasticpolyurethanes available from BASF that exhibit excellent low temperatureproperties and hydrolysis resistance. These products can be injectionand blow molded and extruded. Some grades are suitable for injectionmolding. When compounded with an appropriate co-agent, Elastollan™ maybe crosslinked using irradiation. Other commercially available TPUs,such as Urepan™, may be used when combined with an appropriate co-agent,such as Liquiflex™, a hydroxyl terminated polybutadiene available fromPetroflex.

In general, the thermoplastic polyurethane does not crosslink duringmolding, but may be crosslinked subsequent to molding by applying energyfrom a suitable source. Numerous ways are known to induce crosslinkingin a polymer by free radical initiation, including peroxide initiationand irradiation. In some examples the TPU is crosslinked by irradiation,such as by gamma rays or ultraviolet (UV) irradiation. Other forms ofparticle irradiation, such as electron beam also may be used. The typeof irradiation may be selected based on such factors as the compositionof the underlying layers. For example, certain types of irradiation maydegrade windings in a wound golf ball. On the other hand, balls with asolid core would not be subject to the same concerns. Some types ofirradiation may tend to crosslink (and thus harden) the core. Anappropriate source of irradiation may be selected depending upon whethersuch an effect is desired.

A photoinitiator typically is added to facilitate crosslinking by lightenergy, e.g., UV radiation. Non-limiting examples of UV initiatorsinclude ketones such as 1-hydroxycyclohexylphenylketone,2,2-dimethoxy-1,2-diphenylethan-1-one,1-[4-(2-hydroxyethoxy)-phenyl]-2-methyl-1-propanone (HHPMP), and(bis)acylphosphine-oxides such asbis(2,4,6-trimethylbenzoyl)-phenyl-phosphoneoxide (BTPPO). The amount ofphotoinitiator typically ranges from about 0.1 to about 4 percent byweight of the composition, more usually from about 0.2 to about 2percent by weight.

The cover may also contain other components in addition to thecrosslinkable thermoplastic polyurethane. For example, one or morelayers of the cover may be formed of a composition including very lowmodulus ionomers (VLMIs). As used herein, the term “very low modulusionomers,” refers to ionomer resins that further include a softeningcomonomer X, commonly a (meth)acrylate ester, present from about 10weight percent to about 50 weight percent in the polymer. VLMIs arecopolymers of an α-olefin, such as ethylene, a softening agent, such asn-butyl-acrylate or iso-butyl-acrylate, and an α,β-unsaturatedcarboxylic acid, such as acrylic or methacrylic acid, where at leastpart of the acid groups are neutralized by a magnesium cation or othercation. Other examples of softening comonomers include n-butylmethacrylate, methyl acrylate, and methyl methacrylate. Generally, aVLMI has a flexural modulus from about 2,000 psi to about 10,000 psi.VLMIs are sometimes referred to as “soft” ionomers.

Ionomers, such as acid-containing ethylene copolymer ionomers, includeE/X/Y copolymers where E is ethylene, X is a softening comonomer such asacrylate or methacrylate present in 0 to 50 weight percent of thepolymer, and Y is acrylic or methacrylic acid present in 5 to 35 (often10 to 20) weight percent of the polymer, wherein the acid moiety isneutralized 1 to 90 percent (usually at least 40 percent) to form anionomer by a cation such as lithium, sodium, potassium, magnesium,calcium, barium, lead, tin, zinc or aluminum, or a combination of suchcations, lithium, sodium and zinc being the most preferred. Specificacid-containing ethylene copolymers include ethylene/acrylic acid,ethylene/methacrylic acid, ethylene/acrylic acid/n-butyl acrylate,ethylene/methacrylic acid/n-butyl acrylate, ethylene/methacrylicacid/iso-butyl acrylate, ethylene/acrylic acid/iso-butyl acrylate,ethylene/methacrylic acid/n-butyl methacrylate, ethylene/acrylicacid/methyl methacrylate, ethylene/acrylic acid/methyl acrylate,ethylene/methacrylic acid/methyl acrylate, ethylene/methacrylicacid/methyl methacrylate, and ethylene/acrylic acid/n-butylmethacrylate.

To aid in the processing of the cover stock, ionomer resins may beblended in order to obtain a cover having desired characteristics. Forthis reason, the cover may be formed from a blend of two or more ionomerresins. The blend may include, for example, a very soft material and aharder material. Ionomer resins with different melt flow indexes areoften employed to obtain the desired characteristics of the cover stock.SURLYN® 8118, 7930 and 7940 have melt flow indices of about 1.4, 1.8,and 2.6 g/10 min., respectively. SURLYN® 8269 and SURLYN® 8265 each havea melt flow index of about 0.9 g/10 min. A blend of ionomer resins maybe used to form a cover having a melt flow index, for example, of fromabout 1 to about 3 g/10 min. The cover layer may have a Shore Dhardness, for example, ranging from about 45 to about 70.

As another example, a thermoset cast polyurethane may be used. Thermosetcast polyurethanes are generally prepared using a diisocyanate, such as2,4-toluene diisocyanate (TDI), methylenebis-(4-cyclohexyl isocyanate)(HMDI), or para-phenylene diisocyanate (“PPDI”) and a polyol which iscured with a polyamine, such as methylenedianiline (MDA), or atrifunctional glycol, such as trimethylol propane, or tetrafunctionalglycol, such as N,N,N′,N′-tetrakis(2-hydroxpropyl)ethylenediamine. Othersuitable thermoset materials include, but are not limited to, thermoseturethane ionomers and thermoset urethane epoxies. Other examples ofthermoset materials include polybutadiene, natural rubber, polyisoprene,styrene-butadiene, and styrene-propylene-diene rubber.

When the cover includes more than one layer, e.g., an inner cover layerand an outer cover layer, various constructions and materials aresuitable. For example, an inner cover layer may surround theintermediate layer with an outer cover layer disposed thereon or aninner cover layer may surround a plurality of intermediate layers. Whenusing an inner and outer cover layer construction, the outer cover layermaterial may be a thermoset material that includes at least one of acastable reactive liquid material and reaction products thereof, asdescribed above, and may have a hardness from about 30 Shore D to about60 Shore D.

The inner cover layer may be formed from a wide variety of hard (e.g.,about 65 Shore D or greater), high flexural modulus resilient materials,which are compatible with the other materials used in the adjacentlayers of the golf ball. The inner cover layer material may have aflexural modulus of about 65,000 psi or greater. Suitable inner coverlayer materials include the hard, high flexural modulus ionomer resinsand blends thereof, which may be obtained by providing a cross metallicbond to polymers of monoolefin with at least one member selected fromthe group consisting of unsaturated mono- or di-carboxylic acids having3 to 12 carbon atoms and esters thereof (the polymer contains 1 to 50percent by weight of the unsaturated mono- or di-carboxylic acid and/orester thereof). More particularly, such acid-containing ethylenecopolymer ionomer component includes E/X/Y copolymers where E isethylene, X is a softening comonomer such as acrylate or methacrylatepresent in 0-50 weight percent of the polymer, and Y is acrylic ormethacrylic acid present in 5-35 weight percent of the polymer, whereinthe acid moiety is neutralized about 1-90 percent to form an ionomer bya cation such as lithium, sodium, potassium, magnesium, calcium, barium,lead, tin, zinc, or aluminum, or a combination of such cations. Specificexamples of acid-containing ethylene copolymers include ethylene/acrylicacid, ethylene/methacrylic acid, ethylene/acrylic acid/n-butyl acrylate,ethylene/methacrylic acid/n-butyl acrylate, ethylene/methacrylicacid/iso-butyl acrylate, ethylene/acrylic acid/iso-butyl acrylate,ethylene/methacrylic acid/n-butyl methacrylate, ethylene/acrylicacid/methyl methacrylate, ethylene/acrylic acid/methyl acrylate,ethylene/methacrylic acid/methyl acrylate, ethylene/methacrylicacid/methyl methacrylate, and ethylene/acrylic acid/n-butylmethacrylate.

Non-limiting examples of other suitable inner cover materials that maybe present include thermoplastic or thermoset polyetheresters,polyetheramides, or polyesters, dynamically vulcanized elastomers,functionalized styrene-butadiene elastomers, metallocene polymers,polyamides such as nylons, acrylonitrile butadiene-styrene copolymers(ABS), and blends thereof.

The crosslinkable TPU may be irradiated using any appropriate energysource, such as commercially available UV radiation sources. Because ofthe spherical shape of the golf ball, it is desirable to use devicesthat are capable of applying UV radiation to three dimensional surfaces.The level of radiation may be selected in accordance with the desiredend characteristics of the cover. In general, higher levels of radiationand/or longer exposure times result in a higher degree of crosslinking(e.g., increased hardness), while lower levels of radiation and/orshorter exposure times result in a lower degree of crosslinking (e.g.,less hardness and more elasticity). Dosage levels may vary over a widerange, but by way of example often may range up from about 1 to about 14Mrads, more usually from about 2 to about 12 Mrads. In general, theamount of energy (and thus degree of crosslinking) may be controlled byadjusting one or more of the bulb type, exposure time, filtration, andexposure distance.

The composition of the cover layer and level of radiation may beselected to give a desired hardness, for example a Shore D hardnessranging from about 45 to about 75, often from about 50 to about 70. Ingeneral, crosslinking typically results in increasing Shore D hardnessof the cover by 1-5 units as compared to the hardness beforecrosslinking.

Topcoat

The outer surface of the golf ball typically is painted with at leastone clear or pigmented basecoat primer followed by at least oneapplication of a clear topcoat. The clear topcoat may serve a variety offunctions, such as protecting the cover material, improving aerodynamicsof ball flight, preventing yellowing, and/or improving aesthetics of theball.

One common topcoat utilizes a solvent borne two-component polyurethane,which is applied to the exterior of a golf ball. This topcoatformulation generally requires the use of a solvent that is asignificant source of volatile organic compounds (VOC), which poseenvironmental and health concerns. Ultraviolet (UV) curable coatingsgenerally do not require solvents. As described in co-pending andcommonly owned U.S. patent application Ser. No. 12/470,820, the topcoatmay be applied using a nitrogen- or nitrogen-enriched air deliverysystem. This pending U.S. patent application is entirely incorporatedherein by reference.

Non-limiting examples of topcoats include thermoplastics, thermoplasticelastomers such as polyurethanes, polyesters, acrylics, low acidthermoplastic ionomers, e.g., containing up to about 15% acid, and UVcurable systems. Additional additives optionally may be incorporatedinto the coating material, such as flow additives, mar/slip additives,adhesion promoters, thickeners, gloss reducers, flexibilizers,cross-linking additives, isocyanates, or other agents for toughening orcreating scratch resistance, optical brighteners, UV absorbers, and thelike. The amount of such additives usually ranges from 0 to about 5 wt%, often from 0 to about 1.5 wt %. The thickness of the topcoattypically ranges from of about 5 to about 25 μm, and in some examplesranges from about 10 to about 15 μm.

As described below, in some examples in which a UV curable topcoat isapplied, the crosslinkable TPU in the cover composition and the UVcurable topcoat may be simultaneously exposed to UV radiation such thatthe TPU is crosslinked and the topcoat is cured in a single step.

Manufacturing Process

Golf balls may be formed using a variety of techniques, such asinjection molding, compression molding, retractable pin injectionmolding, vacuum forming, reaction injection molding, liquid injectionmolding, flow coating, and the like. One common technique formanufacturing golf balls is a laminate process. In order to formmultiple layers around the center, a laminate is first formed. Thelaminate includes at least two layers and sometimes includes threelayers. The laminate may be formed by mixing uncured core material to beused for each layer and calendar rolling the material into thin sheets.Alternatively, the laminate may be formed by mixing uncured intermediatelayer material and rolling the material into sheets. The laminate sheetsmay be stacked together to form a laminate having three layers, usingcalender rolling mills. Alternatively, the sheets may be formed byextrusion.

A laminate also may be formed using an adhesive between each layer ofmaterial. For example, an epoxy resin may be used as adhesive. Theadhesive should have good shear and tensile strength, for example, atensile strength over about 1500 psi. The adhesive often has a Shore Dhardness of less than about 60 when cured. The adhesive layer applied tothe sheets should be very thin, e.g., less than about 0.004 inchesthick.

Preferably, each laminate sheet is formed to a thickness that isslightly larger than the thickness of the layers in the finished golfball. Each of these thicknesses can be varied, but all usually have athickness of less than about 0.1 inches. The sheets should have veryuniform thicknesses.

The next step in the method is to form multiple layers around thecenter. This may be accomplished by placing two laminates between a topmold and a bottom mold. The laminates may be formed to the cavities inthe mold halves. The laminates then may be cut into patterns that, whenjoined, form a laminated layer around the center. For example, thelaminates may be cut into figure 8-shaped or barbell-like patterns,similar to a baseball or a tennis ball cover. Other patterns may beused, such as curved triangles, hemispherical cups, ovals, or otherpatterns that may be joined together to form a laminated layer aroundthe center. The patterns may then be placed between molds and formed tothe cavities in the mold halves. A vacuum source often is used to formthe laminates to the mold cavities so that uniformity in layer thicknessis maintained.

After the laminates have been formed to the cavities, the centers arethen inserted between the laminates. The laminates are then compressionmolded about the center under conditions of temperature and pressurethat are well known in the art. The mold halves usually have vents toallow flowing of excess layer material from the laminates during thecompression molding process. As an alternative to compression molding,the core and/or intermediate layer(s) may be formed by injection moldingor other suitable technique.

The next step involves forming a cover around the golf ball core. Thecore, including center and intermediate layers, may be supported withina pair of cover mold-halves by a plurality of retractable pins. Theretractable pins may be actuated by conventional means known to those ofordinary skill in the art.

After the mold halves are closed together with the pins supporting thecore, the cover material is injected into the mold in a liquid statethrough a plurality of injection ports or gates, such as edge gates orsub-gates. With edge gates, the resultant golf balls are allinterconnected and may be removed from the mold halves together in alarge matrix. Sub-gating automatically separates the mold runner fromthe golf balls during the ejection of the golf balls from mold halves.

The retractable pins may be retracted after a predetermined amount ofcover material has been injected into the mold halves to substantiallysurround the core. The liquid cover material is allowed to flow andsubstantially fill the cavity between the core and the mold halves,while maintaining concentricity between the core and the mold halves.The cover material is then allowed to solidify around the core, and thegolf balls are ejected from the mold halves and subjected to finishingprocesses, including topcoating, painting, and/or other finishingprocesses, including processes in accordance with examples of thisinvention, as will be described in more detail below.

B. Specific Examples of Invention

With reference to FIG. 2, golf balls having a crosslinkable TPU in thecover layer may be molded according to techniques previously describedto form a golf ball preform, and then removed from the mold. The golfball preform may be stored for a period of time if desired. The shelflife of the preform (uncrosslinked golf ball) may vary depending on thecompositions of the various layers, processing conditions, and the like.In most cases, the uncrosslinked golf balls may be stored up to aboutone year or more without any adverse affects. In some cases, precautionsshould be taken to minimize light and/or high temperature exposureduring storage. After the targeted characteristics for the golf ballsare identified, the golf balls may be subjected to an energy source,such as UV irradiation as previously described, to achieve the desiredproperties and performance characteristics for the golf balls.Optionally, indicia may be applied to the golf balls at that point,using known techniques, to identify the properties and performancecharacteristics. For example, a corresponding model designation may beaffixed to the golf balls after the cover layer is crosslinked.

Post-mold crosslinking of the TPU in the cover layer offers a number ofadvantages. For example, as schematically illustrated in FIG. 3, golfballs may be first molded at a central manufacturing site and thendistributed to regional suppliers, wholesalers, or possibly even furtherdown the distribution chain such as to retailers or end users. The golfballs may be then stored for period of time (e.g., at least one day, oneweek, one month, 2-10 months, one year, or longer), until the demand forparticular performance characteristics (e.g., distance, spin, hardness,etc.) for golf balls may be assessed for that region or location. Thesupplier, wholesaler, retailer, end user etc. may then irradiate thegolf balls with an energy source, e.g., a commercially available UVenergy source as previously described, under prescribed conditions toselectively crosslink the TPU to achieve the desired properties andperformance characteristics. This provides the benefit of quicklyadapting to changes in preferences or demand for particularcharacteristics of golf balls.

In some instances, the (uncrosslinked) golf ball preforms may be shippedto a third party prior to the step of irradiating. For example, asupplier, distributor, etc. may ship the preforms to a third party whocarries out the irradiating step. In some examples, the golf ballpreform may be packaged (e.g., in sleeves of 2, 3, or 4 balls, and/or inboxes of 12, 15, 18, or 24 balls) prior to irradiation. The golf ballpreforms may be unpacked by a downstream entity (e.g., regional seller,wholesaler, retailer, ultimate customer, etc.), and then irradiated toget the properties desired by the consumer. Optionally, the balls may berepackaged after the irradiation step. Indicia may be applied to thegolf balls and/or packaging material according to properties imparted tothe golf balls by the irradiation step.

In another aspect, the irradiation may be used to simultaneously treatone or more other components of the golf ball in addition to thecrosslinkable TPU in the cover layer. For example, as schematicallyillustrated in FIG. 4, golf balls may be prepared having a crosslinkableTPU in the cover layer and a UV curable topcoat. Exposure of the golfballs to UV radiation under appropriate conditions may affectcrosslinking of the TPU in the cover layer, as well as curing of the UVcurable topcoat. Other components of the golf ball, such as the coreand/or intermediate layer(s), may also contain materials which areaffected by the radiation source. For example, some materials used inthe core may be hardened when exposed to UV radiation.

While the invention has been described in detail in terms of specificexamples including presently preferred modes of carrying out theinvention, those skilled in the art will appreciate that there arenumerous variations and permutations of the above described systems andmethods. Thus, the spirit and scope of the invention should be construedbroadly as set forth in the appended claims.

1. A method of preparing a multi-piece golf ball comprising: providing acore layer; providing a cover layer, wherein the cover layer comprisesat least one crosslinkable thermoplastic polyurethane; molding the coverlayer and core layer to form a golf ball preform in a mold; removing thegolf ball preform from the mold; and irradiating the golf ball preformwith an energy source under conditions sufficient to crosslink thecrosslinkable thermoplastic polyurethane.
 2. The method of claim 1further comprising applying at least one intermediate layer between thecore layer and the cover layer.
 3. The method of claim 2 wherein the atleast one intermediate layer comprises one or more dynamicallyvulcanized thermoplastic elastomers, functionalized styrene-butadieneelastomers, thermoplastic rubbers, thermoset elastomers, thermoplasticurethanes, metallocene polymers, thermoset urethanes, ionomer resins, orblends thereof.
 4. The method of claim 1 wherein the cover layer furthercomprises a material selected from the group consisting of ionomer,thermoplastic, elastomer, urethane, balata, polybutadiene, andcombinations thereof.
 5. The method of claim 1 wherein the core layer isformed from a base composition comprising polybutadiene and about 20 to50 parts of a metal salt diacrylate, dimethacrylate, ormonomethacrylate.
 6. The method of claim 5 wherein the base compositionfurther comprises natural rubber, styrene butadiene, isoprene, orcombinations thereof.
 7. The method of claim 1 wherein the energy sourcecomprises ultraviolet radiation.
 8. The method of claim 7 wherein thecover layer further comprises a UV initiator selected from the groupconsisting of 1-hydroxycyclohexylphenylketone,2,2-dimethoxy-1,2-diphenylethan-1-one,1-[4-(2-hydroxyethoxy)-phenyl]-2-methyl-1-propanone (HHPMP), andbis(2,4,6-trimethylbenzoyl)-phenyl-phosphoneoxide (BTPPO).
 9. A methodof preparing a multi-piece golf ball comprising: preparing a golf ballpreform by molding a cover layer and a core layer to form the golf ballpreform in a mold, wherein the cover layer comprises at least onecrosslinkable thermoplastic polyurethane; removing the golf ball preformfrom the mold; storing the golf ball preform for at least one day;determining demand for golf ball properties, performancecharacteristics, or both; and irradiating the golf ball preform with anenergy source under conditions sufficient to crosslink the crosslinkablethermoplastic polyurethane according to the determined golf ballproperties, performance characteristics, or both.
 10. The method ofclaim 9 wherein the energy source comprises ultraviolet radiation. 11.The method of claim 10 wherein the cover layer further comprises a UVinitiator selected from the group consisting of1-hydroxycyclohexylphenylketone, 2,2-dimethoxy-1,2-diphenylethan-1-one,1-[4-(2-hydroxyethoxy)-phenyl]-2-methyl-1-propanone (HHPMP), andbis(2,4,6-trimethylbenzoyl)-phenyl-phosphoneoxide (BTPPO).
 12. Themethod of claim 9 further comprising distributing the golf ball preformto at least one of a regional supplier, a wholesaler, a retailer, and anend user, prior to the step of irradiating.
 13. The method of claim 9further comprising applying indicia to the irradiated golf ballaccording to the golf ball properties, performance characteristics, orboth.
 14. A method of preparing a multi-piece golf ball comprising:providing a core layer; providing a cover layer, wherein the cover layercomprises at least one crosslinkable thermoplastic polyurethane; moldingthe cover layer and core layer to form a golf ball preform in a mold;removing the golf ball preform from the mold; applying an ultravioletcurable topcoat composition to the golf ball preform; and irradiatingthe golf ball preform with ultraviolet radiation under conditionssufficient to crosslink the crosslinkable thermoplastic polyurethane andcure the topcoat composition.
 15. The method of claim 14 furthercomprising applying at least one intermediate layer between the corelayer and the cover layer.
 16. The method of claim 14 wherein the atleast one intermediate layer comprises one or more dynamicallyvulcanized thermoplastic elastomers, functionalized styrene-butadieneelastomers, thermoplastic rubbers, thermoset elastomers, thermoplasticurethanes, metallocene polymers, thermoset urethanes, ionomer resins, orblends thereof.
 17. The method of claim 14 wherein the cover layerfurther comprises a material selected from the group consisting ofionomer, thermoplastic, elastomer, urethane, balata, polybutadiene, andcombinations thereof.
 18. The method of claim 14 wherein the core layeris formed from a base composition comprising polybutadiene and about 20to 50 parts of a metal salt diacrylate, dimethacrylate, ormonomethacrylate.
 19. The method of claim 18 wherein the basecomposition further comprises natural rubber, styrene butadiene,isoprene, or combinations thereof.
 20. The method of claim 14 whereinthe cover layer further comprises a UV initiator selected from the groupconsisting of 1-hydroxycyclohexylphenylketone,2,2-dimethoxy-1,2-diphenylethan-1-one,1-[4-(2-hydroxyethoxy)-phenyl]-2-methyl-1-propanone (HHPMP), andbis(2,4,6-trimethylbenzoyl)-phenyl-phosphoneoxide (BTPPO).